Epoxy resins are monomers or pre-polymers that react with curing agents, through the epoxy functional ring, to yield high performance cured resins. Such resins, for example, are widely utilized as: protective coatings for electrical insulation; composite matrix resins; and, as structural adhesives, due to their combination of desired chemical and physical characteristics, such as thermal and chemical resistance, adhesion retention and abrasion resistance.
Epoxy resins generally include a plurality of epoxy or oxirane groups. The epoxy groups can react to form a network, typically either through homopolymerization or through addition polymerization with an epoxy curing agent. As used herein, the term "epoxy curing agent" is meant to refer to an agent (or mixture of agents) having three or more reactive sites available for reaction with oxirane groups. As a result of such a structure, an epoxy curing agent can generate a network; i.e. a significantly cross-linked system.
Epoxy curing agents are to be distinguished from compounds referred to herein as merely chain extension agents. As used herein, the term "chain extension agent" is meant to refer to a compound which has only 2 sites capable of reaction with oxirane groups. During polymerization, a chain extension agent will typically become lodged between epoxy resin chains, extending same. Little cross-linking occurs, however, since the chain extension agent does not include a third reactive site.
As used herein, the term "catalyst" is meant to refer to a compound capable of catalyzing polymerization of a di-epoxy resin-compound with substantial networking or cross-linking. Generally, this occurs through generation of anionic or cationic polymerization reactions, typically involving the oxirane moiety. During polymerization in the presence of a catalyst, a di-epoxy compound is capable of reacting at four sites, and thus substantial cross-linking can result.
An example of an epoxy curing agent is a diprimary amine, which is capable of reacting with four epoxy groups. Typical chain extension agents include diphenols, such as resorcinol or bisphenol A. Catalysts include Lewis acids, tertiary amines and imidazoles.
Throughout this specification, "catalysts" and "epoxy curing agents" will be referred to collectively as "epoxy curatives" or "curatives".
Frequently, it is desired that the cured product have a relatively high glass transition temperature (Tg). The glass transition temperature is the temperature at which the cured resin changes from a relatively strong, high modulus, hard, vitreous state to a low modulus, pliable, elastic state. In general, if it is intended that the cured resin be strong at relatively high temperatures, then a relatively high glass transition temperature will be necessary.
A commonly used method of obtaining an improved or higher glass transition temperature is through preparation of a cured resin having a high concentration or degree of cross-linking, or a relatively high concentration of polar groups. A method of achieving high cross-linking is to use an epoxy curing agent having a high level of functionality, or an active homopolymerization agent. In U.S. Pat. No. 4,331,582, incorporated herein by reference, it is taught that bis[4-(N,N-diglycidylamino)phenyl]methane (TGDDM) may be cured with di(4-aminophenyl)sulfone (DDS), to yield a cured resin having a high cross-link density.
Resins having a high cross-link density have several shortcomings. For example, such materials are typically very brittle, and thus undesirable for many applications. That is, the materials are not very tough or ductile. Also, especially if a high concentration of polar groups is utilized to help obtain high glass temperature, the cured polymer may not be satisfactorily stable to moisture.
Generally, to obtain a relatively tough cured resin, it is desired to utilize a composition which exhibits a high degree of cure, and for which, following curing for a reasonably short period of time, a very high percentage of epoxy resin will have reacted to form extended chains within the polymer network. Generally, a high concentration of chain extension agent, such as diphenol, can be utilized to accomplish a high degree of cure. Examples are indicated in U.S. Pat. Nos. 2,934,521 and 3,056,762, the disclosures of which are incorporated herein by reference. A problem with such conventional uses of chain extension agents is that while the resulting resins exhibit a relatively high degree of curing and toughness or ductility, generally the glass transition temperature for the cured product is relatively low, because of low cross-link density.
A substituted fluorene, in particular 9,9-bis(4,4'-hydroxyphenyl)fluorene, is known to react with conventional epoxy polymers, see for example Holloway, Jeffrey G., Low Flammability Epoxy Polymers Via 9,9-Bis(4,4'-Aminophenyl)Fluorene, p. 14, Master's Thesis, San Jose State U. (1984), incorporated herein by reference. A class of compounds which include the above-named substituted fluorene is used, as described below, in preferred embodiments of the present invention, to yield advantages in certain resin compositions.
What has been needed has been a readily curable epoxy resin composition for providing a cured resin having both high glass transition temperature and improved toughness or ductility; i.e. achievement of high glass transition temperature without high cross-link density or polarity which may cause brittleness and/or instability to water. Preferably, the desired features are attainable in a resin composition readily cured by a readily available and effective agent. Also, methods have been needed whereby: improved or higher glass transition temperature for a cured resin composite can be generated without substantial loss of toughness; and/or, improved or higher toughness can be obtained without substantial lowering of glass transition temperature.